Power assisted steering gears are well known, particularly for motor vehicles, as comprising a valve member mounted in a valve sleeve member or a transmission member and rotationally displaceable relative thereto in response to a steering input to adjust control ports of the valve member. Adjustment of the control ports serves to control flow of fluid under pressure from an appropriate power source to power assistance means which would be included in a steering system incorporating the gear and would usually be in the form of a double acting piston and cylinder or ram device. Usually the relative displacement between the valve member and valve sleeve member or transmission member is restricted or resiliently restrained.
It is a consideration in the design of steering gears to ensure that the dimensions of the steering unit are reduced to a minimum which is practical and without loss of efficiency. By this not only will the manufacturing costs of the gear be reduced but the space which is necessary to accommodate the gear and steering system generally will be reduced and this is regarded as most desirable to facilitate the layout of a steering system for, and its accommodation in, a modern vehicle.
For valve components which are rotationally adjustable with respect to each other and in response to a steering input it is important, for efficient operation of the control ports between those components, to ensure that the axial positioning of the components with respect to each other is maintained constant and also that the components are restrained from axial displacement in a housing of the gear. Generally this restraint to axial displacement of the valve components is achieved by use of axial thrust bearings for each of those components. Not only are such thrust bearings relatively expensive but also their accommodation within the housing increases the volume of the housing and therefore the overall size of the gear.
It is an object of the present invention to provide a power assisted steering gear which comprises a valve member which is rotationally adjustable with respect to a valve sleeve member or a transmission member and which includes a compact arrangement for restraining axial displacement of the valve member relative to the member to which it is relatively rotatable and permits a general reduction in the overall size of the gear.